1. Field of the Invention
The present invention relates to a semiconductor device using a crystalline semiconductor (inclusive of a single crystal and a non-single crystal) formed on an insulating substrate such as a glass substrate, quartz substrate, silicon wafer, and the like, and to a method of manufacturing the same. More particularly, it relates to a case of constructing a CMOS circuit by using a n-channel type and a p-channel type semiconductor device in a complementary combination.
2. Description of the Prior Art
Recently, a technique for manufacturing a thin film transistor (TFT) on an inexpensive glass substrate is rapidly advancing. This rapid progress is caused of the growing demand on active matrix display devices. A display device of an active matrix(-addressing) type comprises pixels in a matrix-like arrangement, and a TFT (pixel TFT) is provided to each of the pixels to control the data signal individually by using the switching function of each of the pixel TFTs.
The gate signals and data signals sent to the pixel TFTs thus provided in a matrix-like arrangement are controlled by the peripheral drive circuit formed on the same substrate. A generally prevailed technique for manufacturing a CMOS circuit, i.e., a circuit in which a n-channel TFT and a p-channel TFT are combined in a complementary arrangement, is employed in constructing such a control circuit.
Further, in constructing the peripheral drive circuit described above, a circuit TFT capable of high speed operation is required. Accordingly, a crystalline silicon film is mainly used for the active layer. Because a carrier in a crystalline silicon film moves more rapidly than in an amorphous silicon film, a TFT having superior electric characteristics can be implemented by using the crystalline silicon film.
In this case, FIG. 1A is the cross sectional view of an example of a CMOS circuit constructed from top-gate type TFTs. Referring to FIG. 1A, a base film 102 is formed on the surface of a glass or quartz substrate 101. The structure also comprises a crystalline silicon film for an active layer 103 for a N-channel TFT, as well as another crystalline silicon film for an active layer 104 for a P-channel TFT.
The active layers described above are covered by a gate insulating film 105, and gate electrodes 106 and 107 are formed thereon. The gate electrodes 106 and 107 are further covered by an interlayer insulating film 108 which electrically insulates the gate electrode from the take out line.
Further, source electrodes 109 and 110 as well as a drain electrode 111, which are electrically connected to the active layers 103 and 104 via contact holes, are provided on the interlayer insulating film 108. Because the present case refers to a CMOS circuit, the drain electrode 111 is common for the n-channel TFT and the p-channel TFT. Finally, the source and the drain electrodes 109 to 111 are covered by a protective film 112 to provide a CMOS circuit as shown in FIG. 1A.
The structure shown in FIG. 1A is the simplest constitution of a CMOS circuit, and is an inverter which functions as a circuit for reversing the polarity of a signal. NAND circuit, NOR circuit, and far more complicated logic circuits can be realized by combining such simple CMOS circuits. Various types of electric circuits are designed in this manner.
However, as disclosed in Japanese Laid-open Patent Application No. 4-206971 and Japanese Laid-open Patent Application No. 4-286339, the CMOS circuits manufactured by using a crystalline silicon film suffered a problem that the electric characteristics of the n-channel TFT tend to shift in the direction of depression, whereas that of the p-channel TFT tend to shift in the direction of enhancement.
The electric characteristics (Id–Vg characteristics) of the TFT in the above case is shown in FIG. 1B. In FIG. 1B, the abscissa (Vg) shows the gate voltage, and the ordinate (Id) shows the drain current. The curve indicated by 113 shows the Id–Vg characteristics of the n-channel TFT, and that indicated by 114 shows the Id–Vg characteristics of the p-channel TFT.
The fact that the Id–Vg characteristics 113 of the n-channel TFT shift to the direction of depression and that the Id–Vg characteristics 114 of the p-channel TFT shift to the direction of enhancement both signify that, as shown in FIG. 1B, they are deviated to the negative side with respect to the gate voltage Vg.
Thus, it can be seen that the Id–Vg characteristics 113 and 114 of the n-channel and p-channel TFTs are asymmetrical with respect to the gate voltage of 0 V, and the absolute value of the threshold voltage of the n-channel TFT and that of the p-channel TFT become greatly differed from each other.
However, as disclosed in Japanese Laid Open Patent application No. 4-206971, a deviation in the output voltage due to the difference in the threshold voltage (drive voltage) of the n-channel TFT and that of the p-channel TFT is the cause of decreasing operation speed or malfunction of the CMOS circuit.
To overcome the above problems, the references described above disclose a method of controlling the threshold voltage by adding an impurity element to impart single conductivity to the channel region of the TFTs.
Still, however, in the technique described above (referred to hereinafter as “channel doping”), the control of the quantity of addition was found difficult with decreasing quantity to a trace amount. To the experimental knowledge of the present inventors, no change in threshold value was observed to a quantity of addition of about 1×1018/cm3, but upon exceeding the value, an abrupt change in threshold value was observed for a minute change in concentration.
For instance, in case the shift to be controlled in the threshold voltage is 1 V or lower, a shift in the order of several tenths of volts is realized by an extremely small amount of addition.
Thus, to control the threshold value with high precision, it was found indispensable to precisely control the concentration of the impurity element. However, the delicate control of the impurity element is technically very difficult. For instance, according to the experimental experience of the present inventors, no change in threshold value was observed to a quantity of addition of about 1×1018/cm3, but upon exceeding the value, an abrupt change in threshold value was observed for a minute change in concentration.